Mirrored Object Not being Mirrored
The axis of symmetry used by the mirror modifier is the object's origin (orange dot).  

If  your object has the origin at the center of the object the mirror modifier's effect will not be visible.

Go into edit mode, displace the geometry on the desired axis to one side of the origin, and the mirror modifier's effect will be noticeable.

Pay attention also to the which axis the modifier is acting on.